1. Field of the Invention
The present invention relates to a photovoltaic module and a method for fabricating the same, and particularly relates to a thin film solar cell module of see-through type and a method for fabricating the same.
2. Description of Related Art
Solar energy is a renewable energy, which causes no pollution. It has been the focus in the development of environmental-friendly energy as an attempt to counter the problems such as pollution and shortage of fossil fuels. Herein, solar cells can be used to directly convert solar energy into electrical energy, which becomes a very important research topic now.
Currently, mono-silicon and poly-silicon cells account for more than 90% of the solar cell market. However, manufacturing these types of solar cells requires silicon chips with thickness of 150˜350 micrometers, which increases the production costs. Furthermore, the raw material of solar cells is high-purity silicon ingot. Due to the significant increase in the consumption of silicon ingot, it is being depleted by day. Hence, thin film solar cells have become the new direction in the research and development of solar energy. Thin film solar cells are suitable for mass production and have the advantages of lower production costs and simpler module fabricating process.
FIG. 1 schematically illustrates a conventional thin film solar cell module. As shown in FIG. 1, a thin film solar cell module 150 comprises a glass substrate 152, a transparent electrode 154, a photoelectric conversion layer 156, and a metal electrode 158. Herein, the transparent electrode 154 is disposed on the glass substrate 152. The photoelectric conversion layer 156 is disposed on the transparent electrode 154 by position displacement. In addition, the metal electrode 158 is disposed on the photoelectric conversion layer 156 by position displacement and is in contact with the transparent electrode 154 underneath. In the thin film solar cell module 150, the photoelectric conversion layer 156 usually includes a p-i-n structure composed of a p-type semiconductor, an intrinsic semiconductor, and an n-type semiconductor. Light is transmitted through the bottom of the glass substrate 152 and is absorbed by the photoelectric conversion layer 156 to generate electron-hole pairs. The electron-hole pairs are then separated by an electric field established across the device to form a voltage and an electric current, which are transmitted by a conductive wire for loading. To enhance the efficiency of the cells, in the conventional thin film solar cell module 150, pyramid-like structures or textured structures (not shown) are formed on the surface of the transparent electrode 154 to reduce the reflection of light. The photoelectric conversion layer 156 is usually formed by using an amorphous silicon thin film. However, the band gap of the amorphous silicon thin film is usually between 1.7 eV and 1.8 eV, which merely absorbs sunlight of wavelength less than 800 nm. To increase the utility of light, usually a layer of micro-crystalline (or nano-crystalline) thin film is stacked on the amorphous silicon thin film to form a p-i-n/p-i-n tandem solar cell. The band gap of micro-crystalline (or nano-crystalline) is usually between 1.1 eV and 1.2 eV, which absorbs sunlight of wavelength less than 1100 nm.
In the early times, it was costly and difficult to manufacture solar cells, and solar cells were only used in special fields such as astronautics. Now solar cells, which feature converting solar energy into electric energy, have become more widely used and applied. The applications of solar cells range from the use in apartments and high-rise buildings to that in camper vans and portable refrigerators. However, silicon wafer solar cells are not suitable for certain applications such as transparent glass curtains and buildings integrated with photovoltaic (BIPV). Thin film solar cells of see-through type are used in the aforesaid applications because they are energy-efficient and pleasing to the eye. Further, they accommodate more readily with our living demands.
Currently, techniques related to the thin film solar cells of see-through type and the methods for fabricating the same have been disclosed in some U.S. patents.
U.S. Pat. No. 6,858,461 (U.S. Pat. No. 6,858,461 B2) provides a partially transparent photovoltaic module. As shown in FIG. 2, a photovoltaic module 110 includes a transparent substrate 114, a transparent conductive layer 118, a metal electrode 122, and a photoelectric conversion layer disposed between the transparent conductive layer 118 and the metal electrode 122. Similarly, light is transmitted through the bottom of the transparent substrate 114. In the photovoltaic module 110, a laser scribing process is performed to remove a portion of the metal electrode 122 and a portion of the photoelectric conversion layer to form at least one groove 140 so as to achieve partial transparency of the photovoltaic module 110. However, the laser scribing process is performed at a high temperature. Due to such a high temperature, the metal electrode 122 easily forms metal residues or melts down and accumulates in the grooves, resulting in short circuits of the top and bottom electrodes. On the other hand, an amorphous silicon photoelectric conversion layer recrystallizes at such a high temperature and forms low resistant micro-crystalline (or nano-crystalline) silicon on the sidewalls of the grooves. Consequently, current leakage is increased, and the process yield and the efficiency of the solar cells are affected. In addition, pyramid-like structures or textured structures are usually formed on the surface of the transparent conductive layer 118 to enhance the efficiency of the cells. However, light transmittance is not effectively enhanced because the light transmitted through the bottom of the transparent substrate 114 would be scattered.
In view of the above, to achieve a certain level of light transmittance, larger portions of the metal electrode and the photoelectric conversion layer in a solar cell need to be removed. Please refer to Table 1, which lists the technical specifications of various thin film cells of see-through type manufactured by MakMax Taiyo Kogyo (Japan). According to Table 1, to increase light transmittance, larger portions of the metal electrode and the photoelectric conversion layer need to be removed to decrease the maximum output, efficiency, and fill factor (FF).
TABLE 1TypeKN-38KN-45KN-60Size (mm)980 × 950980 × 950980 × 950Transmittance Rate (%)105<1Maximum Power Output (W)38.045.058.0Vpm (V)58.664.468.0Ipm (A)0.6480.6990.853Voc (V)91.891.891.8Isc (A)0.9721.0901.140Efficiency4.14.86.2FF0.430.450.55
In addition, a photovoltaic device is disclosed in U.S. Pat. No. 4,795,500 (U.S. Pat. No. 4,795,500). As shown in FIG. 3, a photovoltaic device includes a transparent substrate 1, a transparent conductive layer 3, a photoelectric conversion layer 4, a metal electrode 5, and a photoresist layer 8. In the photovoltaic device, holes 6 are formed in the metal electrode 5, the photoelectric conversion layer 4, and even in the transparent conductive layer 3 to achieve transparency. Nevertheless, this patent utilizes a lithographic process which requires expensive facilities and increases the production costs. Additionally, if this patent utilizes a laser scribing process to directly form the holes 6, the problems of metal residue contamination and short circuit will occur to affect the process yield.